Storage batteries or secondary batteries have two general classifications, lead-acid and nickel-alkaline. Their basic feature is that they are rechargeable. The electrochemical action is reversible so that the battery may be charged by passing a current through the battery in the opposite direction to that of discharge.
In the lead-acid type storage battery, the electrolyte is an acid and the plates are largely lead. Normally the electrolyte is a diluted sulfuric acid.
In the alkaline type storage battery, electrical energy is converted from chemical action through an alkaline solution. One type of battery has positive plates with an active material of nickel and negative plates of iron. Another type uses nickel and cadmium plates. A third type uses plates respectively of silver oxide and zinc. In all three types, the electrolyte is potassium hydroxide.
The voltage produced by a storage battery is dependent on its chemistry. In other words, which ever type battery is selected, its voltage output is fixed by the chemicals therein. For example, a lead-acid storage battery produces an output of two volts.
A particular item with which the battery is used on the other hand, may require a battery having a lower voltage. For example, many consumer products use "D" batteries. These are of the primary or non-rechargeable type and produce a voltage output of 1.5 volts. A rechargeable lead-acid battery would not be usable, as such, because its output is two volts.